Multiple CQI feedback for cellular networks

ABSTRACT

Single user and multiuser MIMO transmission in a cellular network may be performed by a base station (eNB) transmitting either one or two transmission layers. A user equipment (UE) receives a reference symbol from the base station. The UE processes the reference symbol with one or more of a plurality of preceding matrices to form a plurality of channel quality indices (CQI). The UE provides feedback to the eNB comprising one or more feedback CQI selected from the plurality of CQI and one or more preceding matrix indicators (PMI) identifying the one or more preceding matrices used to form each of the one or more feedback CQIs, wherein at least two PMI are included in the feedback.

CLAIM TO PRIORITY UNDER 35 U.S.C. 119

This present application is a Continuation of application Ser. No.12/819,182 filed Jun. 19, 2010, which claims priority to an incorporatesby reference U.S. provisional application No. 61/218,688 filed on Jun.19, 2009, entitled “CQI Feedback for Downlink Multiuser MIMO (MU-MIMO)Systems.”

FIELD OF THE INVENTION

This invention generally relates to wireless cellular communication, andin particular to multi-input multi-output transmission in orthogonal andsingle carrier frequency division multiple access (OFDMA) (SC-FDMA)systems.

BACKGROUND OF THE INVENTION

Wireless cellular communication networks incorporate a number of mobileUEs and a number of NodeBs. A NodeB is generally a fixed station, andmay also be called a base transceiver system (BTS), an access point(AP), a base station (BS), or some other equivalent terminology. Asimprovements of networks are made, the NodeB functionality evolves, so aNodeB is sometimes also referred to as an evolved NodeB (eNB). Ingeneral, NodeB hardware, when deployed, is fixed and stationary, whilethe UE hardware may be portable.

User equipment (UE), also commonly referred to as a terminal or a mobilestation, may be a fixed or mobile device and may be a wireless device, acellular phone, a personal digital assistant (PDA), a wireless modemcard, and so on. Uplink communication (UL) refers to a communicationfrom the UE to the NodeB, whereas downlink (DL) refers to communicationfrom the NodeB to the UE. Each NodeB contains radio frequencytransmitter(s) and the receiver(s) used to communicate directly with theUE, which may move freely around it. Similarly, each UE contains radiofrequency transmitter(s) and the receiver(s) used to communicatedirectly with the NodeB. In cellular networks, the UE cannot communicatedirectly with each other but have to communicate with the NodeB.

Long Term Evolution (LTE) wireless networks, also known as EvolvedUniversal Terrestrial Radio Access (E-UTRA), are being standardized bythe 3GPP working groups (WG). OFDMA (orthogonal frequency divisionmultiple access) and SC-FDMA (single carrier FDMA) access schemes werechosen for the down-link (DL) and up-link (UL) of E-UTRA, respectively.User equipment are time and frequency multiplexed on a physical uplinkshared channel (PUSCH), and a fine time and frequency synchronizationbetween UE's guarantees optimal intra-cell orthogonality. In case the UEis not UL synchronized, it uses a non-synchronized Physical RandomAccess Channel (PRACH), and the Base Station provides back someallocated UL resource and timing advance information to allow the UE totransmit on the PUSCH. The general operations of the physical channelsare described in the EUTRA specifications, for example: “3^(rd)Generation Partnership Project; Technical Specification Group RadioAccess Network; Evolved Universal Terrestrial Radio Access (E-UTRA);Physical Channels and Modulation (TS 36.211 Release 8, or later).”

Several types of physical channels are defined for the LTE downlink. Onecommon characteristic of physical channels is that they all conveyinformation from higher layers in the LTE stack. This is in contrast tophysical signals, which convey information that is used exclusivelywithin the physical (PHY) layer. Currently, the LTE DL physical channelsare as follows: Physical Downlink Shared Channel (PDSCH), PhysicalBroadcast Channel (PBCH), Physical Multicast Channel (PMCH), PhysicalControl Format Indicator Channel (PCFICH), Physical Downlink ControlChannel (PDCCH), and Physical Hybrid ARQ Indicator Channel (PHICH).

A reference signal (RS) is a pre-defined signal, pre-known to bothtransmitter and receiver. The RS can generally be thought of asdeterministic from the perspective of both transmitter and receiver. TheRS is typically transmitted in order for the receiver to estimate thesignal propagation medium. This process is also known as “channelestimation.” Thus, an RS can be transmitted to facilitate channelestimation. Upon deriving channel estimates, these estimates are usedfor demodulation of transmitted information. In downlink transmission,two types of reference signals are available. The first type ofreference signal is un-precoded and is transmitted over the entiresystem bandwidth of a cell, and is generally referred to ascell-specific reference signal (CRS). Another type of reference signalis modulated by the same precoder as applied on the data channel, andtherefore enables a UE to estimate the effective precoded MIMO channelcharacteristics. This type of RS is sometimes referred to asDe-Modulation RS or DMRS. DMRS is transmitted only when a UE is beingscheduled, and is therefore only transmitted over the frequency resourceassignment of data transmission. Note that DMRS can also be applied inuplink transmission (PUSCH), in case UE transmitter is equipped withmultiple antennas. Note that RS can also be transmitted for otherpurposes, such as channel sounding (SRS), synchronization, or any otherpurpose. Also note that Reference Signal (RS) can be sometimes calledthe pilot signal, or the training signal, or any other equivalent term.

The LTE PHY can optionally exploit multiple transceivers and antenna atboth the base station and UE in order to enhance link robustness andincrease data rates for the LTE downlink. Spatial diversity can be usedto provide diversity against fading. In particular, maximal ratiocombining (MRC) is used to enhance link reliability in challengingpropagating conditions when signal strength is low and multipathconditions are challenging. Transmit diversity can be used to improvesignal quality by transmitting the same data from multiple antennas tothe receiver. Spatial multiplexing can be used to increase systemcapacity by carrying multiple data streams simultaneously from multipleantennas on the same frequency. Spatial multiplexing may be performedwith one of the following cyclic delay diversity (CDD) precodingmethods: zero-delay, small-delay, or large-delay CDD. Spatialmultiplexing may also be referred to as MIMO (multiple input multipleoutput).

With MRC, a signal is received via two (or more) separateantenna/transceiver pairs. The antennas are physically separated, andtherefore have distinct channel impulse responses. Channel compensationis applied to each received signal within the baseband processor beforebeing linearly combined to create a single composite received signal.When combined in this manner, the received signals add coherently withinthe baseband processor. However, the thermal noise from each transceiveris uncorrelated, resulting in improved signal to noise ratio (SNR). MRCenhances link reliability, but it does not increase the nominal systemdata rate since data is transmitted by a single antenna and is processedat the receiver via two or more receivers. MRC is therefore a form ofreceiver diversity rather than more conventional antenna diversity.

MIMO, on the other hand, does increase system data rates. This isachieved by using multiple antennas on both the transmitting andreceiving ends. In order to successfully receive a MIMO transmission,the receiver must determine the channel impulse response from eachtransmitting antenna. In LTE, channel impulse responses are determinedby sequentially transmitting known reference signals from eachtransmitting antenna. While one transmitter antenna is sending thereference signal, the other antenna is idle. Once the channel impulseresponses are known, data can be transmitted from both antennassimultaneously. The linear combination of the two data streams at thetwo receiver antennas results in a set of two equations and twounknowns, which is resolvable into the two original data streams.

Physical channels are mapped to specific transport channels. Transportchannels are service access points (SAPs) for higher layers. Eachphysical channel has defined algorithms for bit scrambling, modulation,layer mapping, preceding, and resource assignment. Layer mapping andpreceding are related to MIMO applications. Basically, a layercorresponds to a spatial multiplexing channel. Channel rank can varyfrom one up to the minimum of number of transmit and receive antennas.For example, given a 4×2 system, i.e., a system having four transmitantennas and two receive antennas, the maximum channel rank is two. Thechannel rank associated with a particular connection varies in time andfrequency as the fast fading alters the channel coefficients. Moreover,the channel rank determines how many layers, also referred to as thetransmission rank, can be successfully transmitted simultaneously. Forexample, if the channel rank is one at the instant of the transmissionof two layers, there is a strong likelihood that the two signalscorresponding to the two layers will interfere so much that both of thelayers are erroneously detected at the receiver. In conjunction withprecoding, adapting the transmission to the channel rank involvesstriving to use as many layers as the channel rank. Layer mappingspecifies exactly how the extra transmitter antennas are employed. Fornon-codebook based precoding, the precoding applied for the demodulationreference signal (DMRS) is the same as the one applied for the PUSCH(for uplink) and PDSCH (for downlink). Multiplexing of the demodulationreference signals can be time-division multiplexing, frequency divisionmultiplexing, code division multiplexing or a combination of them.

Precoding is used in conjunction with spatial multiplexing. The basicprinciple involved in precoding is to mix and distribute the modulationsymbols over the antennas while potentially also taking the currentchannel conditions into account. Precoding can be implemented by, forexample, multiplying the information carrying symbol vector containingmodulation symbols by a matrix which is selected to match the channelbased on a certain selection criterion. Some examples of selectioncriterion include average throughput and maximumsignal-to-interference-noise ratio (SINR). Sequences of symbol vectorsthus form a set of parallel symbol streams and each such symbol streamis referred to as a “layer”. Thus, depending on the choice of precoderin a particular implementation, a layer may directly correspond to acertain physical antenna or a layer may, via the precoder mapping, bedistributed onto several physical antennas.

In LTE Rel-8, single layer beamforming on antenna port 5 is alreadysupported. Single-layer beamforming is based on non-codebook precodingand relies on a dedicated demodulation reference symbol (DMRS) for datademodulation. DMRS symbols are precoded with the same precoding matricesas the PDSCH data symbols and therefore enable UE to estimate the“effective” channel after precoding. Rank-1 transmission is enforced. AUE is restricted to receive a single transport block (codeword) which ismapped to one layer (data stream) in DL transmission. From the UE'sperspective, the effective 1-layer channel appears as if data istransmitted from a single virtual antenna. DMRS corresponding to thislayer is defined as antenna port 5 in LTE Rel-8 to enable channelestimation.

BRIEF DESCRIPTION OF THE DRAWINGS

Particular embodiments in accordance with the invention will now bedescribed, by way of example only, and with reference to theaccompanying drawings:

FIG. 1 is a pictorial of an illustrative telecommunications network inwhich an embodiment of the invention is used to support single user andmultiuser MIMO transmission signals;

FIG. 2 is an illustrative format of one subcarrier (tone) of a DLtransmission subframe for use in the network of FIG. 1;

FIG. 3 is a flow diagram illustrating enhanced CQI feedback in MU-MIMOoperation in the network of FIG. 1;

FIG. 4 is a flow diagram illustrating enhanced CQI feedback in CoMPoperation in the network of FIG. 1; and

FIG. 5 is a block diagram illustrating an exemplary portion of acellular network with a base station in communication with a mobiledevice.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

A UE that can best estimate channel conditions and then signal the bestcoding to use will get the best performance out of the channel. Althoughthe use of a codebook for precoding limits the best fit to the channel,it significantly simplifies the channel estimation process by the UE andthe amount of uplink signaling needed to convey the desired precoding.Multi-user MIMO (MU-MIMO) and coordinated multi-point (CoMP)transmission complicate the process of providing channel feedback.Various embodiments of an improved feedback protocol for use withMU-MIMO or with CoMP applications will be described in this disclosure.

For best operation, a transmitter must have knowledge of the channel,which is provided by the UE on the uplink control channel. This processis generally referred to as channel station information (CSI) feedback.This knowledge may include a channel quality index (CQI), a precodingmatrix Indicator (PMI), and a rank indication (RI). CSI feedback(RI/PMI/CQI) are recommended MIMO transmission properties derived at theUE based on the channel estimation. For example, RI is the recommendednumber of transmission layers (rank). PMI reflects the recommendedprecoding matrices within the recommended rank (RI). Cal is the observedchannel quality indicator (e.g. recommended modulate and coding scheme)assuming that the RI/PMI feedback are used for MIMO precoding. The PMIfeedback uses a codebook approach to provide an index into apredetermined set of precoding matrices. For 2×2 MIMO, there may bethree different codewords for rank-2 precoding, and four differentcodewords for rank-1 precoding; for 4×2 there may be 16 codewords forrank-1 and rank-2, respectively. Since the channel is continuallychanging, sub-band CQI and PMI information may be provided for multiplepoints across the channel bandwidth, at regular time intervals, up toseveral hundred times a second. The RI is typically provided at a muchlower rate for the whole channel.

The general procedure for determining and specifying CQI and PMI isdefined in the EUTRA specifications, for example: “3^(rd) GenerationPartnership Project; Technical Specification Group Radio Access Network;Evolved Universal Terrestrial Radio Access (E-UTRA); Physical layerprocedures (TS 36.213 Release 8, or later),” which is incorporated byreference herein.

FIG. 1 is a pictorial of an illustrative telecommunications network 100in which an embodiment of the invention is used to support single userand multiuser MIMO transmission signals, as described in more detailbelow. The illustrative telecommunications network includes eNBs 101,102, and 103, though in operation, a telecommunications network mayinclude many more eNBs or fewer eNBs. Each of eNB 101, 102, and 103 isoperable over corresponding coverage areas 104, 105, and 106. Each eNB'scoverage area is further divided into cells. In the illustrated network,each eNB's coverage area is divided into three cells. Handset or otherUE 109 is shown in Cell A 108, which is within coverage area 104 of eNB101. Transmission occurring between eNB 101 and UE 109 via downlinkchannel 110 and uplink channel 112. As UE 109 moves 116 out of Cell A108, and into Cell B 107, UE 109 may be “handed over” to eNB 102.

When UE 109 is not up-link synchronized with eNB 101, non-synchronizedUE 109 employs non-synchronous random access (NSRA) to requestallocation of up-link 112 time or frequency or code resources. If UE 109has data ready for transmission, for example, traffic data, measurementsreport, tracking area update, etc., UE 109 can transmit a random accesssignal on up-link 112 to eNB 101. The random access signal notifies eNB101 that UE 109 requires up-link resources to transmit the UE's data.ENB 101 responds by transmitting to UE 109, via down-link 110, a messagecontaining the parameters of the resources allocated for UE 109 up-linktransmission along with a possible timing error correction. Afterreceiving the resource allocation and a possible timing adjustmentmessage transmitted on down-link 110 by eNB 101, UE 109 may adjust itstransmit timing, to bring the UE 109 into synchronization with eNB 101,and transmit the data on up-link 112 employing the allotted resourcesduring the prescribed time interval. eNB 101 also sends a downlink grantto UE 109 when the eNB has data to transmit to UE 109. The downlinkgrant specifies one or more resource blocks on which the eNB willtransmit to the UE on downlink 110.

Similarly, UE 117 may communicate with eNB 101 on downlink 111 anduplink 113. eNB 101 may decide send data on DL 110 in SU-MIMO mode to UE109. Alternatively, eNB 101 may decide to send data on DL 110 to UE 109and on DL 111 to UE 117 in MU-MIMO mode, as will be described in moredetail below.

Coordinated Multiple Point (CoMP) transmission has been proposed as apromising technology to meet the spectral efficiency requirement inRel-10. As opposed to conventional cellular network where a single UEreceives data transmission from a single eNB at a time, multiple eNBsmay coordinately design the downlink transmission to a UE simultaneouslyon the same frequency resources. In order to reduce the DRS overhead andassist PDSCH demodulation, DRS from different CoMP transmission pointsshould be configured in the same time/frequency resources. In otherwords, the same frequency domain offset of DRS should be used atdifferent CoMP transmission points.

The basic idea of CoMP is to extend the conventional “singlecell-multiple UEs” system structure to a “multiple cells-multiple UEs”network topology so that the concept of cell edge UE would give way tothat of a UE in the vicinity of cell boundaries being at the center areaof a “super-cell” consisting multiple cells. Hence, UEs in CoMPcommunication mode will get much better service and boosted SNR ifseveral nearby cells work in cooperation, where the involved cells arecalled CoMP cooperating cells.

For CoMP operation, when UE 109 is close to a cell boundary, eNB 101also coordinates with eNB 102 via a backhaul interconnecting network todetermine which resource to allocate to UE 109 so that both eNBs cancooperate to transmit in CoMP manner. Once the UE is synchronized, theeNB may both simultaneously send data and reference symbols (RS) on DLin MIMO mode to UE 109, using DL 110 and 120, and receive data andfeedback on UL 112 and 122, for example.

The current E-UTRA (LTE Rel 8) specification supports up to 1-layerbeamforming on antenna port 5. The eNB applies beamforming on it'stransmit antenna array. The beamforming vector is selected by the eNBand is transparent to the UE, i.e., eNB does not explicitly signal thebeamforming vector to UE via DL control signal. To enable channelestimation and data demodulation, dedicated reference signals need to betransmitted. A dedicated demodulation reference symbol (DRS) is to beprecoded by the same precoding vector applied to PDSCH data whichenables UE to estimate the effective downlink channel for demodulation.The same beamforming vector is applied to both DRS and 1-layer PDSCH.

Enhanced CQI/PMI Feedback for CoMP

Three types of feedback categories have been agreed to by the 3GPPworking group for coordinated multi-point (CoMP) transmission:

1) Explicit channel state/statistical information feedback, in which thefeedback represents the channel as observed by the receiver, withoutassuming any transmission or receiver processing;

2) Implicit channel state/statistical information feedback, in whichfeedback mechanisms that use hypotheses of different transmission and/orreception processing, e.g., CQI/PMI/RI;

3) UE transmission of SRS can be used for CSI estimation at eNBexploiting channel reciprocity.

Both explicit and implicit feedback are more tailored to short-termchannel variation to enable short-term beamforming and interferenceavoidance in CoMP transmission. For CoMP cooperation based on long-term(“fixed”) channel characteristics (e.g. angle of arrival (AoA), angle ofdeparture (AoD)), sounding reference symbol (SRS) based channel stateinformation (CSI) estimation exploiting channel reciprocity could beused without incurring additional feedback formats and overhead.

Explicit Feedback

For each cell in the UE's measurement set that is reported in a givensubframe, one or several channel properties may be reported. Channelproperties include (but are not limited to) the following (T is the cellindex):

1) Channel matrix (Hi)—short term (instantaneous)

-   -   The full matrix Hi, or    -   main eigen component(s) of Hi The prefix “eigen” is the German        word for “innate, distinct, self.” Eigen values, eigen vectors        and eigen spaces are properties of a matrix. For example, if the        action of a matrix on a (nonzero) vector changes its magnitude        but not its direction, then the vector is called an eigen vector        of that matrix.

2) Transmit channel covariance (RI), where Ri=(sum{Hij^(†)Hij})/J, j=0,1, 2, . . . , J−1, (‘j’ is span over time or frequency)

-   -   The full matrix Ri, or    -   main elgen component(s) of Ri

3) Inter-cell channel properties may also be reported.

The feedback may also include a noise- and interference part. Forexample, Interference outside the cells reported by the UE or CoMPtransmission points may be reported. Total receive power (lo) or totalreceived signal covariance matrix may be reported. A covariance matrixof the noise-and-interference may include the full matrix, or just themain elgen component(s).

Implicit Feedback Hypotheses

The UE may form hypotheses based on higher layer signaling and thefeedback may be based on one or a combination of various hypothesis. Thefollowing list is an example, but is not intended to be exclusive.

1) Single vs. Multi user MIMO.

2) Single cell vs. Coordinated transmission. Within coordinatedtransmission, there may be single point coordinated scheduling andcoordinated beamforming (CS/CB) vs. multi-point or joint processing (JP)transmission. Within joint processing CoMP, there may be subsets oftransmission points or subsets of reported cells (Joint Transmission).There may also be CoMP transmission point(s) with dynamic cellselection.3) Transmit precoder (i.e. transmission (tx) weights). In JPtransmission, multiple single-cell or multi-cell PMI may be used tocapture coherent or non-coherent channels across reported cells. InCB/CS transmission, single-cell or multiple single-cell PMIs may be usedto capture channels from the reported cell(s) to the UE. A Transmitprecoder may be based on or derived from the PMI weight. Other types offeedback, such as main multi-cell eigen-component, instead of PMI mayalso be performed.4) Receive processing (i.e. receiver (rx) weights).5) Interference based on particular tx/rx processing.

There may be a need for the UE to convey to the network the hypothesisor hypotheses used, if the hypothesis was not conveyed to the UE byhigher layer signaling. In this case, there may be explicit signaling ofhypothesis to eNB. Alternatively, there may be a semi-static hypothesisconfiguration, such as grouping of hypotheses with explicit signaling ofhypothesis by the eNB to the UE. Additionally or alternatively, aprecoded RS may be used to allow UE to generate refined CQI/RI feedback.

For implicit feedback, a structured codebook may be used, where therank-R codebook at the eNB is denoted as defined in equation (1).E _(R)(W _(1,R) ,W _(2,R) , . . . W _(M,R))  (1)where W_(m,R), m=1, 2, . . . M is the N_(t)×R precoding matrix, M is thesize of the rank-R codebook.Implicit Feedback

Implicit feedback is in the form of recommended RI/PMI/CQI based on aUE's measurement. The UE measurement includes both a channel matrix Hand interference and noise information. Rather than having thisinformation explicitly reported, such information is embedded implicitlyin the recommended RI/PMI feedback where a recommended precodingconfiguration is derived to optimize the downlink (DL) throughput.Whether RI/PMI is derived on the short-term instantaneous channel (H andR) or the long term average channel (H=(sum{Hj^(\)Hj})/J, j=0, 1, 2, . .. , J−1) is part of the UE hypothesis and may be signaled to eNB.Alternatively, it is also possible for an eNB to control thisconfiguration by radio resource control (RRC) signaling.

Implicit feedback is a reflection of processed channel information. Anumber of UE implementation specific parameters which are generallyunknown to the eNB, e.g. receiver processing such as minimum mea-squareerror (MMSE) and/or successive interference cancellation (SIC) areusually embedded in the implicit report, which otherwise would requireadditional signaling. This may assist eNB to perform link adaptationaccordingly and possibly reap the gain of a more advanced MIMO receiver.Such advantage is not captured in the explicit channel feedback.

CQI-based feedback currently in Rel-8 of the 3GPP standard allows moretractable testing for each UE report. This is a desirable property thatis also provided by embodiments of the improved feedback designsdescribed herein for CoMP and MU-MIMO.

For implicit feedback, the overhead remains largely independent with thenumber of transmit and receive antennas because it is not necessary toreport the individual channel component (H and R) for eachtransmit/receive antenna pair. In general, a constant feedback overheadunrelated to the UE-specific RX antenna parameter is desirable.

For CoMP, implicit feedback schemes may be for joint processing or forcoordinated beam forming. For joint processing, a UE reports arecommended RI/PMI for the reporting set assuming coherent ornon-coherent combining. Both joint feedback (e.g. coherent combining)and disjoint feedback (non-coherent combining) are possible. For a jointreport, a UE report as single RI/PMI for the CoMP cooperating set. For adisjoint report, the UE report multiple RI/PM to the cells in the CoMPcooperating set.

For coordinated beamforming, a UE reports the recommended RI/PMI for theserving cell, as well as a set of PMIs for the non-serving cells in thereporting set. The PMIs for the non-serving cells are optimized alongwith the serving-cell PMI to reduce the co-channel interference andimprove the cell coverage/throughput.

Implicit Feedback for CoMP

Assuming three cells are in a cooperating set, the received signal atthe UE for joint processing is given in equation (2) and the receivedsignal for coordinated beamforming/scheduling is given in equation (3),where H_(i) and W_(i) denotes the channel and precoding matrixassociated with the i-th transmitting point.y ₁ =H ₁ W ₁ s ₁ +H ₂ W ₂ s ₁ +H ₃ W ₃ s ₁ +n ₁  (2)y ₁ =H ₁ W ₁ s ₁ +H ₂ W ₂ s ₂ +H ₃ W ₃ s ₃ n ₁  (3)

With disjoint feedback, the UE may report a recommended RI/PMI for eacheNB in the measuring set respectively, hence the feedback overheadincreases linearly with the size of the measurement set. We define oneset of recommended RI/PMI as {W₁, W₂, W₃}, where W_(i) is the PMIfeedback for the i-th coordinating eNB.

In the Rel-8 feedback paradigm, only one set of recommended RI/CQI/PMI{W₁, W₂, W₃} is reported by the UE at each feedback instance. Thenetwork may follow the recommended RI/CQI/PMI when the UE feedback isexpected to be accurate, but also has the freedom to override the UEfeedback.

In order to improve the link adaptation and scheduling flexibility, oneenhancement is to feedback multiple sets of RI/PMI as opposed tofeedback of just the “optimal” set only. For example, the 1^(st) optimalset of RI/PMI configuration for the entire CoMP measurement set, the2^(nd) optimal set of RI/PMI configuration for the entire CoMPmeasurement et, so on and so forth. The associated CQIs need to be fedback as well. The eNB will thus have greater flexibility of deciding onthe actual downlink precoding format and to perform scheduling based onthe multiple available precoding choices.

Naturally the improved scheduling and precoding flexibility comes with acost of higher feedback overhead compared to Rel-8 implicit feedback.However note that Rel-8 feedback is designed with a very low feedbackoverhead in mind, thus a moderate increase in the overhead isunderstandably acceptable.

In an extreme case, a UE may report all the possible PMI combinationsM^(K), where M is the codebook size and K is the number of eNBs in themeasurement set, which would provide the finest channel knowledge. Inpractice, this is likely an overkill and it is expected that reporting afew possible PMI configuration sets (e.g. N<<M^(K)) would exploit mostof the CoMP cooperating gains. Most other PMI combinations would likelyresult in sub-optimal spatial combining and therefore does not warrantadditional feedback bandwidth.

To give an example, assume mode 3-1 PUSCH report with wideband PMI andfrequency-selective CQI with subband size of five resource blocks (5 RB)in 10 MHz bandwidth (50 RB). The feedback overhead with the best N setsof RI/CQI/PMI report is given in Table 1.

TABLE 1 Exemplary feedback overhead with best N sets RI/CQI/PMI RI PMICQI Max 3-bits 4N bits 48N bits 52N + 3 N = 1 Max 3-bits   4 bits   48bits  55 bits N = 2 Max 3-bits   8 bits   96 bits 107 bits . . . N = 16Max 3-bits 4N bits 48N bits 835 bits

In this example, for RI, a single rank is reported to all eNBs in thereporting set. Alternatively different RIs may be feedback to differenteNBs in the measurement set; however, this would incur marginal overheadincrease. For PMI, if only the optimal set PMI is reported (as inRel-8), then N=1 PMI is reported to each eNB in the measurement set.This amounts to a feedback overhead of 4 bits per eNB (assuming size-16codebook in each rank). As an alternative, the best N sets of PMIs maybe reported to allow more flexible precoding and scheduling decision.This leads to a PMI overhead of 4N per eNB per report instance. For CQI,similarly, reporting the best N sets of recommended CQIs yields anoverhead of (4+2*10)*2*N=48N, assuming two CQI reports per subband toallow spatial multiplexing.

For instance, the feedback overhead when reporting N=16 sets ofrecommended PMIs amounts to 835 bits. Given that the feedbackperiodicity should in general be an integer multiple of the low-densityCSI cycle (e.g. 10 ms to maintain low overhead for Rel-10 CSI-RS), theaverage feedback rate amounts to 83.5 kbps per UE if one implicit reportis performed every 10 ms. Furthermore, the UE may adaptively configurethe set of PMI report (N) to reach an appropriate tradeoff betweenfeedback overhead and performance.

Implicit Feedback for MU-MIMO

In this section various embodiments of implicit feedback for single-cellmultiuser MIMO (MU-MIMO) transmission are disclosed. A single eNBtransmits to two or more UEs simultaneously over the same frequencyresources in downlink. However the disclosed algorithm may be extendedto the CoMP MU-MIMO context in a straightforward manner.

Assume that two UEs are configured in the MU-MIMO mode. The receivedsignal at user 1 is given by equation (4).y ₁ =H ₁ W ₁ s ₁ +H ₁ W ₂ s ₂ +n ₁  (4)where the first term on the right side denotes the desired signal, andthe second term denotes the inter-user interference. S₁ is the PDSCHdata transmitted to UE1, S₂ is the PDSCH data transmitted to UE2, and n₁is the noise and interference observed by UE1. H1 is the downlinkchannel from the eNB to UE1. W₁ is the precoding matrix for UE1, and W₂is the precoding matrix for UE2. From UE1's perspective, PDSCHtransmission to UE2 (W₂S₂) appears as interference.

To detect s_(i) at UE 1, a symbol-wise slicing is applied as given by(5).ŝ ₁ =L(v ₁ ′y)  (5)where v₁ is the UE beamforming vector.

For example, the linear minimum mean-squared error (LMMSE) receiver maybe expressed as

$\begin{matrix}{v_{1} = {{argmin}\frac{{v_{1}^{\prime}\left( {H_{1}w_{1}w_{1}^{\prime}H_{1}^{\prime}} \right)}v_{1}}{{v_{1}^{\prime}\left( {{H_{1}w_{2}w_{2}^{\prime}H_{1}^{\prime}} + {N_{o}I}} \right)}v_{1}}}} & (6)\end{matrix}$

The interfering precoding vector w₂ is generally assumed not known at UE1.

Enhanced CQI/PMI Feedback for MU-MIMO

In conventional implicit CSI feedback, UE only reports a single set ofRI/PMI/CQI for its own use (i.e., W₁ as in Eqn [5]). A simple method toextend the Rel-8 feedback structure is to allow multiple sets ofrecommended RI/PMI to be reported, for itself as well as for ahypothetical user to be co-scheduled in the MU-MIMO mode. To be morespecific, in one set of a PMI report, a UE may report a recommendedprecoder PMI₁ (W₁), a recommended interfering precoder PMI₂ (W₂) for theco-scheduler UE, along with the CQI assuming (PMI₁, PMI₂) is used in theDL. Thus, one set of an implicit report is defined as: O=(PMI₁, PMI₂,CQI). Upon receiving the UE feedback, eNB understands that PMI₁ is therecommended preceding matrix for the target UE, while PMI₂ is therecommended preceding matrix for a potentially co-scheduled UE.Subsequently, eNB may compare the PMI feedback (PMI₁, PMI₂) fromdifferent users in order to find out if two UEs can be paired up inMU-MIMO transmission. If the PMI report from two users match each other(e.g., PMI₁ of UE1 matches PMI₂ of UE2, and PMI₂ of UE1 matches PMI₁ ofUE2), eNB may pair these two UEs together in MU-MIMO transmission mode.

Similar to the CoMP scenario, multiple sets of PMI combinations (desiredand interfering) allows an eNB to have more refined knowledge to do linkadaptation of the channel quality when different precoding matrixcombinations are used in the downlink, and allows better UE pairing.Hence, it is possible to report multiple sets of such PMI combinations,e.g.

O₁=(PMI₁₁, PMI₂₁, CQI₁)

O₂=(PMI₁₂, PMI₂₂, CQI₂)

. . .

O_(N)=(PMI_(1N), PMI_(2N), CQI_(N)).

Brute-Force Feedback

Let K denote the total number of UEs that are simultaneously supported(M≧K), then the total number of possible precoding matrix combinations(N) are given by (7).

$\begin{matrix}{{P_{K}^{M} = \frac{M!}{\left( {M - K} \right)!}},} & (7)\end{matrix}$

For each of these possible precoding matrix combinations, there exists aunique CQI for the target UE and it can be accurately calculated. Thisbrute-force method will exhaustively feed back these CQIs to the servingNode B to perform user scheduling and link adaptation. A detailedprocedure is given as follows, where UE 1 is chosen as the desired UEwithout loss of generality.

Let (n,m) denote a precoding matrix combination where n is the PMI of UE1, and m is the PMI of UE 2. n and in are chosen from [1, M] where M isthe codebook size.

For each possible precoding matrix combination (n,m), UE 1 derives itscorresponding CQI. The total number of precoding matrix combinations andCQI is given by M(M−1), when K=2. These M(M−1) CQIs are fed back to theserving Node B, along with the indices of their PMI combinations.

Obviously, this brute-force feedback achieves the performance upperbound with implicit feedback.

Best-N Set Feedback

Alternatively, another embodiment may report the best N sets of CQI/PMI.A few examples of the criterion for selecting the best N sets aredescribed below.

Example 1

The best N sets of PMI combinations which result in the best CQIs forthe desired UE are reported.

Example 2

For each possible precoding matrix n of UE 1 (W₁), ┌N/M┐ precodingmatrices candidates (W₂) are assumed for the co-scheduled UE and thecorresponding CQIs are reported to the UE. The criterion for choosingthe ┌N/M┐ precoding matrices (W₂) for the co-scheduled UE may be suchthat it maximizes the CQI of the desired UE, or to maximize a certaindistance matrix (e.g. Chordal distance or Fubini-norm distance) to theprecoding matrix of the desired UE (W₂).

Example 3

For each possible precoding matrix for user 1 (W₁), ┌N/M┐ precodingmatrices are assumed for the co-scheduled UE and the corresponding CQIsare reported to the UE. The criterion for choosing the ┌N/M┐ precodingmatrices (W₂) may be the precoding matrices whose correlation to theprecoding matrix of the desired UE (W₁) is minimized. Alternatively, forexample, two UEs are allowed to be transmitted at the same time if theirprecoding vectors satisfies|w ₁ ′w ₂|<ρwhere ρ is a pre-defined threshold on the correlation.

Example 4

Assume wideband preceding, frequency-selective CQI with subband size 5RB and 10 MHz system bandwidth (50 RB). As the number of layers for eachUE in MU-MIMO mode has not bee decided, rank-1 transmission (e.g. asingle MCS report) is assumed as aligned with MU-MIMO in Rel-8 andRel-9. Assuming size-16 codebook and a maximum of two UEs in MU-MIMO,the feedback overhead when the best N sets of PMI combinations arereported, is given as2*4N+(4+2*10)*N=32N.

For example, if the best N=16 sets of PMI combinations are reported, thefeedback overhead is 512 bits per feedback instance. If the feedbackperiodicity is equivalent to the channel state information referencesymbol (CSI-RS) cycle (e.g. 10 ms), this amounts to a feedback rate of51 kbps. Alternatively, the feedback overhead may be reduced byconfiguring four or eight sets, for example, of CQI/PMI report N by RRCsignaling.

FIG. 2 is an illustrative format of one subcarrier (tone) of a DLtransmission subframe for use in the network of FIG. 1. It comprises of14 resource elements. Elements of the present invention will bedescribed in the context of EUTRA sub-frame, even though itsapplicability is broader. Orthogonal frequency division multiple access(OFDMA) based systems include classic OFDMA as well as its alternatives,like single carrier frequency division multiple access (SC-FDMA) anddiscrete Fourier transform (DFT)-spread OFDMA. In OFDMA based systems,frequency resources are divided into tones. Tones are further groupedinto “tone blocks” or “resource blocks” for purposes offrequency-dependent scheduling of mobiles, and other possible purposes.Thus, each mobile can be allocated one or more resource blocks in anOFDMA based system. This group of resource blocks will be denoted as thefrequency allocation for a given mobile.

FIG. 2 illustrates just one subcarrier of sub-frame 200 comprising twoslots 201 and 202. It comprises of 14 resource elements. This resourceelement is configured for use on antenna port 7 or 8, as will bedescribed in more detail below. Duration of the EUTRA sub-frame is 1 ms,which means that duration of two slots 201 and 202 is 0.5 ms each. Eachslot comprises seven symbols when a normal cyclic protection field (CP)is appended to each symbol, or six symbols when an extend CP is appendedto each symbol. For example, slot 201 comprises symbols 203-209. Theslot 202 comprises symbols 210-216. Symbols 208, 209, 215 and 216 areDemodulation (DM) Reference symbols (RS), and are used to derive channelestimates which are needed for coherent demodulation of the remainingsymbols that are modulated with payload data. LTE Rel 9 also definesseveral other antenna port configurations for antenna ports 0-3 and 5,where port 0-3 are unprecoded cell-specific reference symbols (CRS)antenna ports and port 5 is DMRS for single-layer data transmissiondefined in Rel-8. Each symbol has a time duration equal to approximatelyT, which is a function of the slot time. In this embodiment, the slottime is 500 μsec. Since the first symbol in the slot has more cyclicprefix samples, not all symbols are exactly equal in duration, as per3GPP 1836.211. Nevertheless, all symbols can be considered to beapproximately equal in duration, which doesn't exceed 75 μsec. Note thatif all symbols were exactly equal in duration, the symbol time T wouldapproximately be equal to 500 μsec/7=71.4 μsec.

In some embodiments of the invention, the set of reference signalsequences comprises CAZAC sequences and near-CAZAC sequences. Near-CAZACis a term which designates sequences which are obtained using computersearch methods, and whose properties approximate CAZAC properties. Insome embodiments of the invention, CAZAC sequences are Zadoff-Chusequences. In some embodiments of the invention, near-CAZAC sequencesare sequences of the form exp(j*π*(φ(n)/4); wherein the length of φ(n)is an integral multiple of 12. Here, “j” is the imaginary unit.

In some embodiments of the invention, the set of reference signalsequences comprises CAZAC sequences only. In some embodiments of theinvention, the set of reference signal sequences comprises near-CAZACsequences only. In some embodiments of the invention, the set ofreference signal sequences comprises both CAZAC sequences and near-CAZACsequences. Sometimes, a phase ramp is applied to modify the firstsequence, for example exp(j*n*α+j*π*φ(n)/4) can still be considered as areference signal sequence. For 3GPP EUTRA, there are 30 possiblesequences of length 24, which are also near-CAZAC. For length 36 andmore, sequences are produced from CAZAC sequences. Thus, the set ofreference signal sequences comprises both CAZAC and near-CAZACsequences.

Further details on the construction of reference signals, demodulationreference signals and sounding reference signals are included in 3rdGeneration Partnership Project; GPP TS 36.211 V9.1.0 (2010) “TechnicalSpecification Group Radio Access Network; Evolved Universal TerrestrialRadio Access (E-UTRA); Physical Channels and Modulation,” in particularin section 6 and which is incorporated herein by reference.

There are currently two definitions of “transparency”. A first type oftransparency is defined in terms of SU/MU-MIMO mode. Non-transparencymeans that a UE is semi-statically configured in either the SU orMU-MIMO mode which is signaled to the UE via higher layer signaling. UEis semi-statically configured to operate in SU-MIMO transmission mode orMU-MIMO transmission mode, hence different DL (downlink) control and UEfeedback are used. Higher layer (L3) radio resource control (RRC)signaling configures the SU/MU-MIMO mode for the UE. Transparency meansthat the UE is configured in one joint SU/MU MIMO transmission mode,thus the same UE feedback and DL control.

A second type of transparency is defined in terms of knowledge of theco-scheduled UE. Non-transparency means that the presence of aco-scheduled UE is known and possibly taken into account in the DLcontrol signaling and UE feedback. On the other hand, non-transparencymeans that UE is agnostic about the presence of a co-scheduled UE, hencethe same UE feedback and DL control is used.

Regardless of which definition is used, “transparency” essentially meansthat no signaling is provided to the UE regarding transmission to theother UEs in the same time/frequency resources. If the presence of aco-scheduled UE is provided to the target UE, MU-MIMO transmission is“non-transparent”. For example, in LTE Rel-8, MU-MIMO is asemi-statically configured transmission mode, hence a UE will know thatit will be paired up with another UE if it is configured in such atransmission mode. Another possibility is to signal to the UEdynamically whether or not transmission to another UE is present, thatis, a dynamic downlink grant has been provided on the PDCCH. In thiscase, a UE can be either configured in the semi-static SU or MU-mode.

FIG. 3 is a flow diagram illustrating enhanced CQI feedback in MU-MIMOoperations. An eNB in a given cell is transmitting 302 to multiple UE inMU-MIMO mode on different beams. As described earlier, the downlinktransmission includes reference symbols (RS) that may be used by the UEto determine channel quality. A first UE, (UE1) receives 304 a referencesymbol from the base station (eNB). Simultaneously, a second UE (UE2)receives 314 the same signal on a different beam.

Each UE processes 306, 316 the reference symbol with one or more of aplurality of precoding matrices to form a plurality of channel qualityindices (CQI) as described in more detail above.

Each UE provides feedback 308, 318 to the eNB comprising one or morefeedback CQI selected from the plurality of CQI and one or moreprecoding matrix indicators (PMI) identifying the one or more precedingmatrices used to form each of the one or more feedback CQIs, wherein atleast two PMI are included in the feedback. For example, the pluralityof CQI is representative of all preceding matrices assigned to the UE,and the entire plurality of CQI and respective precoding matrixindicators are provided as feedback.

In another embodiment, each of the plurality of CQI is formed using adifferent combination of two or more of the plurality of precedingmatrices. The feedback comprises two or more PMI identifying thecombination of precoding matrices used to form each of the one or morefeedback CQI. In particular, each combination of precoding matrices mayinclude at least a first preceding matrix recommended for UE1, and asecond precoding matrix recommended for a hypothetical UE2 to beco-scheduled with the UE. Since UE1 is not aware of UE2 and visa versa,each UE processes the RS and assumes there is a hypothetical UE that isoperating with it in MIMO mode.

As described earlier, processing 306, 316 may calculate a correlationbetween each combination of preceding matrices and select for feedbackonly combinations having a lower correlation. In some embodiments, onlycombinations having a correlation below a selected threshold areselected for feedback.

Alternatively, as described earlier, processing 306, 316 may calculate adistance between each combination of preceding matrices and select forfeedback only combinations having a larger distance.

FIG. 4 is a flow diagram illustrating enhanced CQI feedback in CoMPoperation. In this mode, two or more eNB are transmitting 402, 403 to aUE in CoMP mode on the same RB. As described earlier, the downlinktransmission includes reference symbols (RS) that may be used by the UEto determine channel quality. Depending on the CoMP configuration, allof the cooperating eNB may be transmitting to the UE using exactly thesame resources, or the cooperating eNB may be transmitting to the sameUE using different resources, such as different resource symbols.

In the illustrated embodiment, the UE receives 404 a first referencesymbol (RS1) from eNB1 and also a different RS2 from cooperating eNB2.

The UE processes each of the received reference symbols with one or moreof a plurality of precoding matrices to form a set of channel qualityindices (CQI) for RS1 and another set of CQI for RS2.

The UE provides feedback 408 to the eNB that includes one or morefeedback CQI and PMI selected for the first RS and one or more feedbackCQI and PMI selected for the second RS.

eNB1 receives 410 the feedback and dynamically optimizes transmissionbased on the one or more feedback CQI selected for RS1 and eNB2simultaneously receives 411 the feedback and dynamically optimizestransmission based on the one or more feedback CQI selected for RS2. Ifthere are more than two eNB cooperating, a similar process is performedby each eNB.

System Example

FIG. 5 is a block diagram illustrating an exemplary portion of thecellular network of FIG. 1. As shown in FIG. 5, the wireless networkingsystem 1000 includes a UE device 1001 in communication with an eNB 1002.The UE device 1001 may represent any of a variety of devices such as aserver, a desktop computer, a laptop computer, a cellular phone, aPersonal Digital Assistant (PDA), a smart phone or other electronicdevices. In some embodiments, the electronic UE device 1001 communicateswith the eNB 1002 based on a LTE or E-UTRA protocol. Alternatively,another communication protocol now known or later developed can be used.

As shown, the UE device 1001 includes a processor 1003 coupled to amemory 1007 and a Transceiver 1004. The memory 1007 stores (software)applications 1005 for execution by the processor 1003. The applications1005 could be any known or future application useful for individuals ororganizations. As an example, such applications 1005 could becategorized as operating systems (OS), device drivers, databases,multimedia tools, presentation tools, Internet browsers, e-mailers,Voice-Over-Internet Protocol (VOIP) tools, file browsers, firewalls,instant messaging, finance tools, games, word processors or othercategories. Regardless of the exact nature of the applications 1005, atleast some of the applications 1005 may direct eNB (base-station) 1002to transmit DL signals to UE device 1001 periodically or continuouslyvia the transceiver 1004.

Transceiver 1004 includes uplink logic which may be implemented byexecution of instructions that control the operation of the transceiver.Some of these instructions may be stored in memory 1007 and executedwhen needed. As would be understood by one of skill in the art, thecomponents of the uplink and downlink logic may involve the physical(PHY) layer and/or the Media Access Control (MAC) layer of thetransceiver 1004. Transceiver 1004 includes two or more receivers 1020and two or more transmitters 1022 for SU/MU-MIMO, as described in moredetail above.

eNB 1002 includes a Processor 1009 coupled to a memory 1013 and atransceiver 1010. Memory 1013 stores applications 1008 for execution bythe processor 1009. The applications 1008 could be any known or futureapplication useful for managing wireless communications. At least someof the applications 1008 may direct the base-station to managetransmissions to or from the user device 1001.

Transceiver 1010 includes a resource manager which enables eNB 1002 toselectively allocate uplink PUSCH resources and downlink PDSCH resourcesto the user device 1001. As would be understood by one of skill in theart, the components of the resource manager 1012 may involve thephysical (PHY) layer and/or the Media Access Control (MAC) layer of thetransceiver 1010. Transceiver 1010 includes a Receiver 1011 forreceiving transmissions from various UE within range of the eNB andtransmitter 1014 for transmission to the various UE within range. Theresource manager executes instructions that control the operation oftransceiver 1010. Some of these instructions may be located in memory1013 and executed when needed. The resource manager controls thetransmission resources allocated to each UE that is being served by eNB1002 and broadcasts control information via the physical downlinkcontrol channel PDCCH.

During MIMO transmission from eNB 1002 via transmitters 1014 on PDSCH,eNB 1002 monitors channel conditions to adapt to the prevailingcondition. This includes monitoring the channel quality indicator (CQI)and precoding matrix index (PMI) feedback provided by UE 1001 on theuplink channel using condition monitoring logic 1012 that is coupled toreceiver 1011. As was described in more detail above, the enhancedCQI/PMI feedback includes at least two PMI in various embodiments of theinvention.

During MIMO transmission to UE 1001 via transmitters 1014 on PDSCH, eNB1002 forms DMRS signals using different amounts of cyclic shift,depending on the number of layers being used for transmission, asdescribed in more detail above.

A typical eNB will have multiple sets of receivers and transmitterswhich operate generally as described herein to support hundreds orthousand of UE within a given cell. Each transmitter may be embodiedgenerally by a processor 1009 that executes instructions from memory1013 to perform the scrambling, mapping, and OFDM signal formation,using signal processing techniques as are generally known in the artalong with embodiments of the invention described herein.

Other Embodiments

While the invention has been described with reference to illustrativeembodiments, this description is not intended to be construed in alimiting sense. Various other embodiments of the invention will beapparent to persons skilled in the art upon reference to thisdescription. For example, a larger or smaller number of symbols thendescribed herein may be used in a slot.

While the invention has been described with reference to DLtransmission, it may be equally applied to UL transmission.

Embodiments of the invention may support single user (SU) dual-layerbeamforming using UE specific RS for both LTE-TDD (time division duplex)and FDD (frequency division duplex) using UE specific demodulationreference signals and mapping of physical data channel to resourceelements that may provide forward compatibility with LTE-A DemodulationRS (DMRS). Embodiments of the invention extend single user dual-layerbeamforming to multi-user dual-layer beamforming.

The term “frame” and “subframe” are not restricted to the structure ofFIG. 2. Other configurations of frames and/or subframes may be embodied.In general, the term “frame” may refer to a set of one or moresubframes. A transmission instance likewise refers to a frame, subframe,or other agreed upon quantity of transmission resource.

Embodiments of this invention apply to various types of frequencydivision multiplex based transmission. Thus, the concept can easily beapplied to: OFDMA, OFDM, OFT-spread OFDM, OFT-spread OFDMA, SC-OFDM,SC-OFDMA, MC-CDMA, and all other FDM-based transmission strategies.

A NodeB is generally a fixed station and may also be called a basetransceiver system (BTS), an access point, or some other terminology. AUE, also commonly referred to as terminal or mobile station, may befixed or mobile and may be a wireless device, a cellular phone, apersonal digital assistant (PDA), a wireless modem card, and so on.

As described in general above, an embodiment of the invention mayperform all tasks described herein such as channel monitoring andprecoding selection, formation of transmission signals, etc, using logicimplemented by instructions executed on a processor. Another embodimentmay have particular hardwired circuitry or other special purpose logicoptimized for performing one or more to the tasks described herein.

An embodiment of the invention may include a system with a processorcoupled to a computer readable medium in which a software program isstored that contains instructions that when executed by the processorperform the functions of modules and circuits described herein. Thecomputer readable medium may be memory storage such as dynamic randomaccess memory (DRAM), static RAM (SRAM), read only memory (ROM),Programmable ROM (PROM), erasable PROM (EPROM) or other similar types ofmemory. The computer readable media may also be in the form of magnetic,optical, semiconductor or other types of discs or other portable memorydevices that can be used to distribute the software for downloading to asystem for execution by a processor. The computer readable media mayalso be in the form of magnetic, optical, semiconductor or other typesof disc unit coupled to a system that can store the software fordownloading or for direct execution by a processor.

As used herein, the terms “applied,” “coupled,” “connected,” and“connection” mean electrically connected, including where additionalelements may be in the electrical connection path. “Associated” means acontrolling relationship, such as a memory resource that is controlledby an associated port.

It is therefore contemplated that the appended claims will cover anysuch modifications of the embodiments as fall within the true scope andspirit of the invention.

The invention claimed is:
 1. A method of operating a user equipmentdevice in a cellular network, the method comprising: receiving a firstreference symbol at the user equipment (UE) from a first base station(eNB); processing by the UE the first reference symbol with one or moreof a plurality of precoding matrices to form a plurality of channelquality indices (CQI), wherein the plurality of CQI is representative ofall precoding matrices assigned to the UE, and wherein the entireplurality of CQI and respective precoding matrix indicators are providedas feedback; and providing feedback from the UE to the eNB comprisingone or more feedback CQI selected from the plurality of CQI and one ormore precoding matrix indicators (PMI) identifying the one or moreprecoding matrices used to form each of the one or more feedback CQIs,wherein at least two PMI are included in the feedback, wherein eachcombination of precoding matrices comprise of at least a first precodingmatrix recommended for the UE, and a second precoding matrix recommendedfor a hypothetical UE to be co-scheduled with the UE.
 2. The method ofclaim 1, wherein the plurality of CQI is representative of all precodingmatrices assigned to the UE, and wherein the entire plurality of CQI andrespective precoding matrix indicators are provided as feedback.
 3. Themethod of claim 2, wherein the received feedback comprises combinationsof PMI selected by the UE that have a lower correlation between eachcombination of precoding matrices.
 4. The method of claim 2, wherein thereceived feedback comprises combinations of PMI selected by the UE thathave a larger distance between each combination of precoding matrices.5. The method of claim 3, wherein only combinations having a correlationbelow a selected threshold are received as feedback.
 6. The method ofclaim 1, further comprising receiving a second reference symbol from asecond eNB coincidently with receiving the first reference symbol (RS);wherein the processing comprises forming one or more CQI for each of thefirst RS and the second RS using one or more of a portion of theplurality of precoding matrices; and wherein feedback comprise one ormore feedback CQI and PMI selected for the first RS and one or morefeedback CQI and PMI selected for the second RS.
 7. The method of claim1, wherein the UE is a mobile handset.
 8. The method of claim 1, furthercomprising the first base station receiving feedback from a second UEcomprising one or more feedback CQI selected by the second UE from aplurality of CQI formed by the second UE and one or more precodingmatrix indicators (PMI) identifying the one or more preceding matricesused by the second UE to form each of the one or more feedback CQIs,wherein at least two PMI are included in the feedback, wherein eachcombination of preceding matrices comprise of at least a first precedingmatrix recommended for the second UE, and a second preceding matrixrecommended for a hypothetical UE to be co-scheduled with the UE; andwherein the eNB uses the feedback from the UB and the second UE toselect preceding matrices for use by the eNB for co-scheduledtransmission of data to the UE and to the second LIE in multi-usermulti-input multi-output (MU-MIMO) mode.
 9. A base station apparatus,comprising: circuitry for transmitting a reference symbol from the basestation (eNB) for reception by a user equipment (UE); circuitry forreceiving feedback from the UE comprising one or more feedback CQIselected by the UE from a plurality of CQI formed by the UE and one ormore precoding matrix indicators (PMI) identifying the one or moreprecoding matrices used by the UE to form each of the one or morefeedback CQIs, wherein at least two PMI are included in the feedbackand, wherein each of the plurality of CQI is formed using a differentcombination of two or more of the plurality of precoding matrices, thefeedback comprises two or more PMI identifying the combination ofprecoding matrices used to form each of the one or more feedback CQI,and each combination of precoding matrices comprise of at least a firstprecoding matrix recommended for the UE, and a second precoding matrixrecommended for a hypothetical UE to be co-scheduled with the UE; and aprocessor coupled to the circuitry for transmitting and to the circuitryfor receiving, the processor operable to use the at least two PMI in thefeedback to select a precoding matrix for use by the eNB fortransmission of data to the UE, and wherein the processor is operable touse the feedback from the UE to select precoding matrices for use by theeNB for co-scheduled transmission of data to the UE and to a second UEin multi-user multi-input multi-output (MU MIMO) mode.
 10. The apparatusof claim 9, wherein the processor is operable to use the feedbackcomprising two or more PMI from the UE and from the second UE to selectprecoding matrices for use by the eNB for co-scheduled transmission ofdata to the UE and to the second UE in MU-MIMO mode.
 11. A userequipment apparatus, comprising: circuitry for receiving a firstreference symbol from a first base station (eNB); circuitry forprocessing the first reference symbol with one or more of a plurality ofprecoding matrices to form a plurality of channel quality indices (CQI),wherein each of the plurality of CQI is formed using a combination oftwo or more of the plurality of precoding matrices; and circuitry forproviding feedback to the eNB comprising one or more feedback CQIselected from the plurality of CQI and one or more precoding matrixindicators (PMI) identifying the one or more precoding matrices used toform each of the one or more feedback CQIs, wherein at least two PMI areincluded in the feedback, wherein the feedback comprises two or more PMIidentifying the combination of precoding matrices used to form each ofthe one or more feedback CQI.
 12. The apparatus of claim 11, wherein theplurality of CQI is representative of all precoding matrices assigned tothe UE, and wherein the entire plurality of CQI and respective precodingmatrix indicators are provided as feedback.
 13. The apparatus of claim11, wherein each combination of precoding matrices comprise of at leasta first precoding matrix recommended for the UE, and a second precodingmatrix recommended for a hypothetical U to be co-scheduled with the UE.14. The apparatus of claim 11, further comprising circuitry forcalculating a correlation between each combination of precoding matricesand selecting for feedback only combinations having a lower correlation.15. The apparatus of claim 14, wherein only combinations having acorrelation below a selected threshold are selected for feedback. 16.The apparatus of claim 11, further comprising circuitry for calculatinga distance between each combination of precoding matrices and selectingfor feedback only combinations having a larger distance.
 17. Theapparatus of claim 11, further comprising circuitry for receiving asecond reference symbol from a second eNB coincidently with receivingthe first reference symbol (RS); wherein the processing comprisesforming one or more CQI for each of the first RS and the second RS usingone or more of a portion of the plurality of precoding matrices; andwherein feedback comprise one or more feedback CQI and PMI selected forthe first RS and one or more feedback CQI and PMI selected for thesecond RS.
 18. The apparatus of claim 11, wherein the UE is a mobilehandset.
 19. A method of operating a user equipment device in a cellularnetwork, the method comprising: receiving a first reference symbol atthe user equipment (UE) from a first base station (eNB); processing bythe UE the first reference symbol with one or more of a plurality ofprecoding matrices to form a plurality of channel quality indices (CQI),wherein each of the plurality of CQI is formed using a combination oftwo or more of the plurality of precoding matrices; and providingfeedback from the UE to the eNB comprising one or more feedback CQIselected from the plurality of CQI and one or more precoding matrixindicators (PMI) identifying the one or more precoding matrices used toform each of the one or more feedback CQIs, wherein at least two PMI areincluded in the feedback, wherein the feedback comprises two or more PMIidentifying the combination of precoding matrices used to form each ofthe one or more feedback CQI.
 20. The method of claim 19, wherein eachcombination of precoding matrices comprise of at least a first precodingmatrix recommended for the UE, and a second precoding matrix recommendedfor a hypothetical UE to be co-scheduled with the UE.
 21. The method ofclaim 20, wherein only combinations having a correlation below aselected threshold are selected for feedback.
 22. The method of claim19, further comprising calculating a correlation between eachcombination of precoding matrices and selecting for feedback onlycombinations having a lower correlation.
 23. The method of claim 19,further comprising calculating a distance between each combination ofprecoding matrices and selecting for feedback only combinations having alarger distance.
 24. A method for operating a base station device in acellular network, the method comprising: transmitting a reference symbolfrom the base station (eNB) for reception by a user equipment (UE);receiving feedback from the UE comprising one or more feedback CQIselected by the UE from a plurality of CQI formed by the UE and one ormore precoding matrix indicators (PMI) identifying the one or morepreceding matrices used by the UE to form each of the one or morefeedback CQIs, wherein at least two PMI are included in the feedback,wherein each of the plurality of CQI is formed using a combination oftwo or more of the plurality of precoding matrices; and using the atleast two PMI included in the feedback to select a precoding matrix foruse by the eNB for transmission of data to the UE, wherein the feedbackcomprises two or more PMI identifying the combination of precodingmatrices used to form each of the one or more feedback CQI.
 25. Themethod of claim 24, wherein the plurality of CQI is representative ofall precoding matrices assigned to the UE, and wherein the entireplurality of CQI and respective precoding matrix indicators are receivedas feedback.